Since global environmental regulations have been more stringently enforced lately, it is essentially required that the content of environmental pollutants of the exhaust gas emitted from engines is reduced, and in order to solve the problem, it is necessary to raise the combustion temperature by increasing the explosion pressure of the engine. In order to withstand the explosion pressure when the explosion pressure of the engine is increased as described above, strength of an engine cylinder block and head constituting the engine needs to be increased.
A material currently used as a material for the engine cylinder block and head is flake graphite cast iron to which alloy iron, such as chromium (Cr), copper (Cu), and tin (Sn), is added. The flake graphite cast iron has excellent thermal conductivity and vibration damping and includes a trace of alloy iron, which also has excellent castability as well as low chilling probability. However, since the tensile strength ranges from 150 to 250 MPa, there is a limitation in using the flake graphite cast iron for an engine cylinder block and head, which requires an explosion pressure of more than 180 bar.
Meanwhile, high-strength, such as a tensile strength of approximately 300 MPa, is required for a material for an engine cylinder block and head to withstand an explosion pressure of more than 180 bar. For this purpose, a pearlite stabilizing element such as copper (Cu) and tin (Sn), or a carbide production promoting element such as chromium (Cr) and molybdenum (Mo) needs to be further added, but since the addition of such alloy iron potentially includes the chilling tendency, there is a problem of increasing the likelihood that chills occur at a thin walled part of an engine cylinder block and head having a complicated shape.
The related art for achieving high strength of the flake graphite cast iron is to form an MnS sulfide by controlling the ratio of using manganese (Mn) and sulfur (S) added to the cast iron melt, that is, Mn/S to a specific ratio. In this case, the Mn/S sulfide formed serves to promote the nucleation of graphite and reduce chilling by the addition of alloy iron, and the method may be applied only to the high-manganese cast iron melt, in which the content of manganese (Mn) is approximately from 1.1 to 3.0%. Manganese (Mn) reinforces the matrix structure by promoting the pearlite structure and making cementite spacing in the pearlite structure dense, but when manganese (Mn) is added in a large amount, manganese (Mn) stabilizes the carbide and suppresses the growth of graphite, so that the strength may be increased to 350 MPa or more, but when the Mn/S ratio is not controlled within a specific range, chilling is further promoted and fluidity is rather reduced due to the high content of manganese. Accordingly, there is a limitation in applying the flake graphite cast iron as a material for an engine cylinder block and head having a complicated structure.
Recently, compacted graphite iron (CGI) cast iron simultaneously satisfying high tensile strength of 350 MPa or more while having excellent castability, vibration damping capacity, and thermal conductivity of the flake graphite cast iron has been applied as a material for an engine cylinder block and head having a high explosion pressure. In order to make a CGI cast iron having a tensile strength of 350 MPa or more, high-quality pig iron having low content of impurities such as sulfur (S) and phosphorus (P), and a molten material need to be used, and it is necessary to precisely control magnesium (Mg) which is a graphite-spheroidizing element. However, since it is difficult to control magnesium (Mg) and magnesium is very sensitive to a change in melting and casting conditions, such as a tapping temperature and a tapping rate, it is highly likely that material defects and casting defects of CGI cast iron will occur, and there is a problem in that the costs of production increase.
Since CGI cast iron has relatively worse workability than flake graphite cast iron, when an engine cylinder block and head is manufactured using CGI cast iron, processing is not performed in a processing line dedicated to the existing flake graphite cast iron and it is essentially required that the processing line is changed into a processing line dedicated to CGI cast iron. Therefore, there is a problem concerning the occurrence of enormous facility investment costs.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.